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P. subradiatum is a distinctive species on the vertical Carboniferous limestones of northern and western Ireland. It is occasionally plentiful, forming small, dark brown to almost grey-black, matt, narrow, radiating arc-like crescents (the result of the earlier circular thallus dying away from the centre) on limestone cliffs or beneath low ledges wetted by seepage. In such situations it is often a companion species to the local Collema fragile. It prefers south or south-east-facing rock where heat retention is high.

Key characteristics

The dark, narrow, arc-like thallus on dry limestone is distinctive

Low (<2m high), south-east-facing, tiered cliffs with lip-like overhangs are the best sites to look for it.